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International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VII, Issue XII, December 2018 | ISSN 2278-2540
www.ijltemas.in Page 26
Application of Synchronous Motor to Improve Power
Factor for Industrial Loads Dr. Aung Zaw Latt
Department of Electrical Power Engineering, Technological University (Maubin), Maubin, Myanmar
Abstract— Most of the industrial loads such as induction motors
are inductive in nature and hence have low lagging power factor.
Around 60% of the utility load consists of motors and hence the
overall power factor of the power system is low. The low power
factor is highly undesirable as it causes an increase in current,
resulting in additional losses of active power in all the elements
of power system from power station generator down to the
utilization devices. This paper describes the power factor can be
improved by connecting the synchronous motor operated in an
overexcited mode in parallel with the industrial loads operating
at lagging power factor. The designed industrial load is 8 TPH
rice milling plant consists of unity power factor loads 250 kW
(lighting loads and other loads) and induction motor loads 350
kW, efficiency 96 % at power factor 0.7 lagging. In this paper,
the appropriate rating of synchronous motor is selected to
compensate the reactive power and improve the power factor of
rice milling plant from 0.7 lagging to the region within 0.95
laggings and unity.
Keywords—power factor, power factor improvement, lagging,
leading, unity, synchronous motor, induction motor
I. INTRODUCTION
ormally, the power factor of the whole load on a large
generating station is in the region of 0.8 to 0.9. However,
sometimes it is lower and is such case it is generally desirable
to take special steps to improve the power factor. Low power
factor can increase electricity generating costs, reduce
electrical distribution capacity and increase voltage drop.
Therefore, lower factor is undesirable from economic point of
view. The low power factor is mainly due to the facts that
most of the industrial loads are inductive and, therefore take
lagging currents. In order to improve the power factor,
synchronous motor equipment taking leading power should be
connected in parallel with the load, it takes a leading current which partly neutralizes the lagging reactive component of the
load. Application of synchronous motor as compensation
equipment in a system has the following effects: (1)
Reduction in reactive component of line current. (2)
Maintenance of voltage profile within limit. (3) Reduction of
I2R losses in the line and other equipment due to reduction in
current. (4) Reduction in investment in the system per kW of
load supplied. (5) Decrease in kVA loading of generators,
transformers and other equipments. This decrease in kVA
loading may relieve an over load conditions or release
capacity of load growth. (6) Reduction in kVA demand
charges for large consumers. (7) Improvement in power factor
of the system.
A. Characteristic of synchronous motor
In industry, synchronous motors are used mainly where a
constant speed is desired. An important feature of a synchronous motor is that it can draw either lagging or
leading reactive current from the ac supply system. In a
synchronous motor, if the rotor field winding provides just the
necessary excitation, the stator will draw no reactive current;
that is, the motor will operate at a unity power factor. If the
rotor excitation current is decreased, lagging reactive current
will be drawn from the ac source to aid magnetization by the
rotor field current, and the machine will operate at a lagging
power factor. If the rotor field current is increased, leading
reactive current will be drawn from the ac source to oppose
magnetization by the rotor field current, and the machine will operate at a leading power factor. Thus, by changing the field
current, the power factor of the synchronous motor can be
controlled. If the motor is not loaded, but is simply floating on
the ac supply system, it will thus behave as a variable inductor
or capacitor as its rotor field current is changed. A
synchronous machine with no load is called a synchronous
condenser. It may be used in power transmission systems to
regulate line voltage. In industry, synchronous motors are
used with other induction motors and operated in an
overexcited mode so that they draw leading current to
compensate the lagging current drawn by the induction
motors, thereby improving the overall plant power factor.
3-ø Load
text
text
text
text
text
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3-ø Synchronous motor
IIL
Im I
Φm
ΦΦL
Im
V
IL
Fig.1 Power factor improvement by synchronous motor method
Fig.1 shows the power factor improvement by synchronous
motor method. The three phase load takes current IL at low
lagging power factor cos ΦL. The synchronous condenser
N
International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VII, Issue XII, December 2018 | ISSN 2278-2540
www.ijltemas.in Page 27
takes a current Im which leads the voltage by an angle Φm.
The resultant current I is the phasor sum of Im and IL and lags
behind the voltage by an angle Φ. It is clear that Φ is less than
ΦL so that cos Φ is greater than cos ΦL.Thus the power factor
is increased from cos ΦL to cos Φ.
B. Effect of Change of Excitation of Synchronous Motor
The terminal voltage V of a synchronous motor is constant.
If the mechanical load is constant, a change in excitation (i.e.
field current) causes a change in power factor, because the
phasor relation between E and V (Fig.2) must be satisfied at
all values of E. This characteristic of a synchronous motor makes it as a very suitable device for improvement of power
factor is power systems. A constant applied voltage means a
constant resultant flux. This resultant flux is due to the action
of both dc source and ac source. If excitation is large, the field
produced by rotor is too high. Consequently the armature mmf
becomes magnetizing, so as to keep the resultant flux
constant.
θδ
Ψ
E
V
j Ix
IR
I
Fig.2 Phasor diagram of synchronous motor
I1
I3
I2 V
E3 E2 E1
0
jI1xsjI2xs
j I3xs
θ1
θ3
Fig.3 Effect of excitation on power factor of synchronous motor
Fig. 3 shows the phasor diagram of a synchronous motor
supplying a constant mechanical loat at three different values
of excitation E1, E2 and E3. The power developed is (3 VE/xs)
sin δ. As E changes, δ also changes, so as to keep the power
developed at constant value. The locus on E is the dotted line. When emf is E1 , the field flux is too high. The current I1 is
now at a leading power factor cos θ1. The leading reactive
current exerts a demagnetizing effect to restore the resultant flux to the constant value The phasor sum of E1 and jI1xs gives
the terminal voltage V (the armature resistance Ra has been
neglected). When the excitation emf is E2, there is no excess
flux produced by the rotor winding. Therefore the input
current I2 has no reactive component and the power factor is
1. When the excitation voltage is E3, the motor is under
excited. The armature current now assumes the position I3 and
the power factor is lagging. The lagging current has a
magnetizing effect, which helps to establish the air gap flux
demanded by the terminal voltage. Thus an over excited
synchronous motor operates at a leading power factor, where
as an under excited one operates at a lagging power factor.
C. Synchronous motor with different excitations
A synchronous motor is said to have normal excitation
when Eb =V. If field excitation is such that Eb < V, the motor
is said to under-excited. In both these conditions, it has a if d.c
field excitation is such that Eb > V, then motor is said to be
over-excited and draws a leading current. Fig. 4 shows the
different excitation conditions of synchronous motor.
O
EbER
θ
øα α
Ia
V
EbER
VO
θ
øα α
Ia
(a) Eb=V (lagging P.F) (b) Eb<V (lagging P.F)
O
EbER
V
θøα α
Ia
O
Eb
θα αIa
Ø=0
(c) Eb>V (leading P.F) (d) Eb>V ( Unity P.F)
Fig.4 Synchronous motor with different excitation condition
There will be some value of excitation for which armature
current will be in phase with V, so that power factor will
become unity, as shown in Fig.4 (d). The value of ∞ and back
e.m.f Eb can be found with the help of vector diagrams for
various power factors as shown in Fig.5.
θøα A
Ia
C
αB O V Ia
α αθ
øB
A
C
O
Eb
(a) Lagging P.F (b) Leading P.F
C
θ
O B AIa V
α
(c) Unity P.F
Fig.5 Power factor diagrams of Synchronous motor
International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VII, Issue XII, December 2018 | ISSN 2278-2540
www.ijltemas.in Page 28
II. POWER FACTOR IMPROVEMENT
A. Power Factor and Power Triangle
The cosine of angle between voltage and current in an a.c
circuit is known as power factor. In an a.c circuit, there is
generally a phase difference Φ between voltage and current.
The term cos Φ is called the power factor of the circuit. If the
circuit is inductive, the current lags behind the voltage and the
power factor is referred to as lagging. However, in a
capacitive circuit, current leads the voltage and power factor
is said to be leading. Consider an inductive circuit taking a
lagging current I from supply voltage V; the angle of lag being Φ. The phasor diagram of the circuit is shown in Figure.5.
The circuit current I can be resolved into two perpendicular
components; I cos Φ in phase with V and I sin Φ 90 Hout of
phase with V. The I cos Φ is known as active or wattful
component, where as component I sin Φ is called the reactive
or wattles component. The reactive component is measure of
the power factor. If the reactive component is small, the phase
angle Φ is small and hence power factor cos Φ will be high.
Therefore, a circuit having small reactive current I sin Φ will
have high power factor and vice-versa.
The analysis of power factor can also made in terms of
power drawn by the a.c circuit. If each side of the current triangle oab of Fig.6 (a) is multiplied by voltage V, then we
get the power triangle OAB shown in Fig.6 (b).
O V
b
ø
I cos ø
II sin ø
a
O
B
ø
VI cos ø
VIVI sin ø
A
(a)Phasor diagram (b) Power Triangle
Fig. 6 Phasor diagram and power triangle of the circuit
OA = VI cos Φ and represents the active power in kW
AB = VI sin Φ and represents the reactive power in kVAR
OB = VI and represents the apparent power in kVA
(kVA)2 = (kW)2 + (kVAR)2 (1)
Cos Φ = active power
apparent power =
kW
kVA (2)
kVAR = kVA sin Φ = kW
cos Φ sin Φ = kW tan Φ (3)
B. Power Factor Correction
The power factor correction can also be illustrated from
power triangle. Thus referring to Fig.7, the power triangle
OAB is for the power factor cos Φ1, where as power triangle
OAC is for the improved power factor cos Φ2. It may be seen
that active power (OA) does not change with power factor
improvement. However, the lagging kVAR of the load is
reduced by the power factor correction equipment, thus
improving the power factor to cos Φ2. Knowing the leading
kVAR supplied by the power factor correction equipment, the
desired results can be obtained.
Leading kVAR supplied by p.f correction equipment
= kVAR1- kVAR2 (4)
= kW (tan Φ1 – tan Φ2) (5)
O
B
kW
C
Aø1 ø2
kVAR2
kVAR1
Fig.7 Power factor correction diagram
III. CASE STUDY OF POWER FACTOR IMPROVEMENT USING
SYNCHRONOUS MOTOR
The following data are the required capacity of the
designed industrial loads of 8 TPH rice milling plant. This
rice milling plant loads are operated with full load.
(i) Lighting loads and others = 250 kW at unity p.f
(ii) Total Induction Motor loads = 350 kW, efficiency 96 % at
p.f 0.7 lagging
In this case study, 160 kW, 180 kW, 200 kW, 225 kW and
250 kW of synchronous motor with efficiency 95 % are used
to calculate and select the appropriate rating of synchronous
motor to improve the power factor 0.7 to 0.95 lagging of rice
milling plant. These various synchronous motors are
considered at 0.75 leading, 0.8 leading, 0.85 leading, and 0.9
leading, 0.95 leading and unity power factor for 50%, 75%,
100% and 125% change of induction motor loads. The power
factor of overall plant is limited within 0.95 lagging and unity.
Using the suffixes 1, 2 and 3 to indicate the different loads,
kVA1 = kW1
cos Φ1 (for unity p.f load) (6)
kVA2 = kW2
cos Φ2 (for induction motor ) (7)
kVA3 = kW3
cos Φ3 (for synchronous motor ) (8)
kVAR1 = kVA1 x sin Φ1 (for unity p.f load) (9)
kVAR2 = kVA2 x sin Φ2 (for induction motor) (10)
International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VII, Issue XII, December 2018 | ISSN 2278-2540
www.ijltemas.in Page 29
kVAR3 = kVA3 x sin Φ3 (for synchronous motor) (11)
+ -
V
kW2
kW3
kW1= kVA1
kVA2
kVAR2kVAR3
kVA3 Φ3Φ2
0
Fig. 8 Power factor improvement diagram for rice milling plant
These loads are represented in Fig.8. The three kVAs’ are
not in phase. In order to fine the total kVA, we resolve each kVA into rectangular components kW and kVAR as shown in
Fig.6. The total kW and kVAR may then combine to obtain
total kVA. kVAR2 and KVAR3 are in the opposite directions:
kVAR2 being a lagging while kVAR3, being a leading kVAR.
TABLE I
PERCENTAGE OF INDUCTION MOTOR LOADS
% of
I.M Load
Real
Power (kW)
Reactive
Power(kVAR)
50 182.5 186.178 (lag;)
75 273.75 278.267 (lag;)
100 365 372.356 (lag;)
125 456.75 465.445 (lag;)
TABLE II
CALCULATED RESULTS USING 160 KW SYNCHRONOUS MOTOR
% of
I.M
loads
p.f of synchronous motor
0.75
(lead)
08
(lead)
0.85
(lead)
0.9
(lead)
0.95
(lead) 1
50 0.998
(lag;)
0.995
(lag;)
0.990
(lag;)
0.985
(lag;)
0.977
(lag;)
0.955
(lag;)
75 0.983
(lag;)
0.976
(lag;)
0.969
(lag;)
0.961
(lag;)
0.951
(lag;)
0.927
(lag;)
100 0.961
(lag;)
0.954
(lag;)
0.946
(lag;)
0.937
(lag;)
0.926
(lag;)
0.903
(lag;)
125 0.940
(lag;)
0.932
(lag;)
0.924
(lag;)
0.916
(lag;)
0.905
(lag;)
0.803
(lag;)
TABLE III
CALCULATED RESULTS USING 180 KW SYNCHRONOUS MOTOR
% of
I.M
loads
p.f of synchronous motor
0.75
(lead)
08
(lead)
0.85
(lead)
0.9
(lead)
0.95
(lead) 1
50 0.999
(lag;)
0.998
(lag;)
0.994
(lag;)
0.989
(lag;)
0.980
(lag;)
0.958
(lag;)
75 0.988
(lag;)
0.982
(lag;)
0.976
(lag;)
0.967
(lag;)
0.957
(lag;)
0.932
(lag;)
100 0.969 0.961 0.953 0.944 0.933 0.908
(lag;) (lag;) (lag;) (lag;) (lag;) (lag;)
125 0.949
(lag;)
0.940
(lag;)
0.932
(lag;)
0.923
(lag;)
0.912
(lag;)
0.888
(lag;)
TABLE IV
CALCULATED RESULTS USING 200 KW SYNCHRONOUS MOTOR
% of
I.M
loads
p.f of synchronous motor
0.75
(lead)
08
(lead)
0.85
(lead)
0.9
(lead)
0.95
(lead) 1
50 0.999
(lag;)
0.998
(lag;)
0.996
(lag;)
0.991
(lag;)
0.984
(lag;)
0.960
(lag;)
75 0.992
(lag;)
0.987
(lag;)
0.980
(lag;)
0.972
(lag;)
0.961
(lag;)
0.935
(lag;)
100 0.975
(lag;)
0.968
(lag;)
0.959
(lag;)
0.950
(lag;)
0.938
(lag;)
0.911
(lag;)
125 0.956
(lag;)
0.948
(lag;)
0.939
(lag;)
0.929
(lag;)
0.918
(lag;)
0.892
(lag;)
TABLE V
CALCULATED RESULTS USING 225 KW SYNCHRONOUS MOTOR
% of
I.M
loads
p.f of synchronous motor
0.75
(lead)
08
(lead)
0.85
(lead)
0.9
(lead)
0.95
(lead) 1
50 0.999
(led;)
0.999
(lag;)
0.998
(lag;)
0.995
(lag;)
0.987
(lag;)
0.963
(lag;)
75 0.996
(lag;)
0.991
(lag;)
0.985
(lag;)
0.978
(lag;)
0.967
(lag;)
0.939
(lag;)
100 0.982
(lag;)
0.975
(lag;)
0.967
(lag;)
0.957
(lag;)
0.945
(lag;)
0.916
(lag;)
125 0.965
(lag;)
0.957
(lag;)
0.948
(lag;)
0.937
(lag;)
0.925
(lag;)
0.896
(lag;)
TABLE VI
CALCULATED RESULTS USING 250 KW SYNCHRONOUS MOTOR
% of
I.M
loads
p.f of synchronous motor
0.75
(lead)
08
(lead)
0.85
(lead)
0.9
(lead)
0.95
(lead) 1
50 0.997
(led;)
0.999
(led;)
0.999
(lag;)
0.996
(lag;)
0.989
(lag;)
0.966
(lag;)
75 0.998
(lag;)
0.995
(lag;)
0.989
(lag;)
0.982
(lag;)
0.971
(lag;)
0.943
(lag;)
100 0.987
(lag;)
0.981
(lag;)
0.973
(lag;)
0.963
(lag;)
0.951
(lag;)
0.921
(lag;)
125 0.972
(lag;)
0.966
(lag;)
0.955
(lag;)
0.944
(lag;)
0.931
(lag;)
0.901
(lag;)
TABLE VII
COMPARISON RESULTS USING VARIOUS SYNCHRONOUS MOTOR
% of
I.M
load
The operating leading power factor of synchronous motor
160 kW 180 kW 200 kW 225 kW 250 kW
50 0.75 ~ 1 0.75 ~ 1 0.75 ~ 1 0.75 ~ 1 0.75 ~ 1
75 0.75 ~ 0.95 0.75 ~ 0.95 0.75 ~ 0.95 0.75 ~ 0.95 0.75 ~ 0.95
100 0.75 ~ 0.8 0.75 ~ 0.85 0.75 ~ 0.9 0.75 ~ 0.9 0.75 ~ 0.9
125
Below
limited
region
Below
limited
region
0.75 0.75~ 0.8 0.75~ 0.85
Using the equations, the result data are calculated as shown
in Table II, III, IV, V and VI. The comparison results by using
International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VII, Issue XII, December 2018 | ISSN 2278-2540
www.ijltemas.in Page 30
different ratings of synchronous motor are shown in Table
VII. According to the results data, the power factor using the
ratings of 225kW and 250kW synchronous motor are
operating within the limit, but they are large and more cost.
The range of power factor by using 200kW synchronous
motor can operate the plant load changing within the limited
region (0.95 lagging and unity). Therefore, the rating of
200kW synchronous motor is an appropriate motor for the
power factor improvement of 8 TPH rice milling plant.
IV. CONCLUSION
In case of low power factor of the system, the system current will be increased and this high current of the system
will cause to the large line losses (copper losses), the large
kVA rating and size of electrical equipments, the greater
conductor size and cost, the poor voltage regulation and large
voltage drop, the low efficiency and penalty from electric
power supply company. Therefore, power factor correction is
becoming a greater concern for industrial and manufacturing
plants all around the world.
In practice, the static capacitor, phase advancer and
synchronous motor are mainly used as the load compensator.
Generally, synchronous motor is used to improve the power
factor in large industries because of synchronous motor has
some advantages in comparison with others compensator. The
power factor of a synchronous motor is controllable within its
design and load limits. According to the results data, this
paper show that the power factor of 8TPH rice milling plant
for 50%, 75%, 100% and 125% change of induction motor
loads can be improved from 0.7 lagging p.f to the limited
region within 0.95 lagging and unity p.f when the plant loads
are connected in parallel with the rating of 200kW
synchronous motor at 0.75 leading, 0.8 leading, 0.85 leading,
and 0.9 leading, 0.95 leading and unity power factor.
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